This invention relates to color picture tubes of the type having a shadow mask attached to a peripheral frame which is suspended in relation to a cathodoluminescent screen, and particularly to a novel mask-frame assembly shape which aids in reducing shadow mask doming during tube operation.
In these color picture tubes, the accuracy with which the electron beams, emitted from an electron gun, strike individual elemental cathodoluminescent screen areas depends, to a great degree, upon the accuracy with which the shadow mask apertures are aligned with the elemental screen areas during operation of the tube. During initial tube operation, the shadow mask is heated by the impingement of the electron beams thereon at a much faster rate than is the heavier peripheral frame. Because of this heating differential, the shadow mask expands at a greater rate than the frame. Since the shadow mask is peripherally welded to the frame, this more rapid expansion of the shadow mask causes the frame to resist the mask expansion, thereby resulting in mask doming. Such mask doming causes the electron beams, passing therethrough, to misregister with their associated elemental screen areas, thereby resulting in an undesirable shift in color on the screen.
Several prior patents have addressed this doming problem. U.S. Pat. No. 3,368,098, issued to Demmy on Feb. 6, 1968, teaches the use of a limited number of mask-to-frame welds to eliminate or reduce a negative distortion of a shadow mask during the warmup period of tube operation. U.S. Pat. No. 3,585,431, issued to Long on June 15, 1971, discloses the formation of a flutted mask skirt, wherein the mask is welded to a frame such as to permit radial movement of the mask away from a peripheral surface of the frame during tube warmup. U.S. Pat. No. 3,862,448, issued to Ishizuka et al. on Jan. 21, 1975, teaches the use of a reduced mass cushion zone at a mask skirt to minimize the stresses in the mask caused by heating the mask. U.S. Pat. No. 4,099,086, issued to Roder et al. on July 4, 1978, discloses a mask skirt having cutouts near its corners to define corner tongues. The tongues are curved and are welded to a mask frame. In one embodiment, the tongues have a further recess or a corrugation to provide a hinge effect. Each of these prior patents is hereby incorporated by reference for purpose of including the disclosures contained therein.
Although all of these prior patents solved particular problems associated with the tube types in which they were applied, more recent tube designs necessitate the development of other solutions to the problems caused by mask expansion. One of these more recent tube designs utilizes a flatter, more truely rectangular, shadow mask.